Where can I get it and flash it?
Assuming you are looking for the MBR hex file? You can find it in the nRF5 SDK: \nRF5_SDK_17.0.2\components\softdevice\mbr\hex
What's it used for?Can I not use it? How to flash it?And what's the MBR data storage?
The MBR is required for DFU. You can read more about what it does in the Master boot record and bootloader section of the Softdevice specification.
